1. Field of the Invention
The invention relates to a process and to an apparatus for facsimile coding in which the items of information contained on a two-color original composed of individual surface elements are scanned for the purpose of transmission, in which sequence lengths are represented by code words which consist of binary characters, and in which a line synchronizing word is produced at the beginning of the coding of each line.
2. Description of the Prior Art
The publication by D. Preuss: Redundanzreduzierende Codierung von Faksimilesignalen, Nachrichtetechnische Zeitschrift (Redundancyreducing Coding of Facsimilie Signals, Communications Technological Magazine), Vol. 11 (1971) pp. 564 to 568 has already disclosed a process for coding the lengths of sequences of a first and a second color in facsimile transmission, which process is referred to as sequence length coding. In this process the lines which are to be scanned are broken up into portions of the same color, or brightness, which are referred to as sequences. For each sequence, a code word is produced which indicates the particular number of surface elements, referred to as sequence length, in the form of a dual number. If one code word is not sufficient to represent a sequence length, further code words of the same length are added to this code word.
In the German DAS 2,335,836 another process for coding facsimile signals is disclosed. In this process the length of the code word of a sequence of a specific color is estimated from the length of at least one preceding sequence of the same color. For example, the length of the code word is estimated on the basis of the length of the preceding sequence of the same color in the line currently being scanned and from the length of the adjacent sequence of the same color in the preceding line.
If a code word produced by this known process is transmitted in faulty fashion, the associated sequence length is incorrectly reproduced in the receiver, and the entire following image content is displaced. Faulty transmission has particularly disadvantageous results when a code word has been adjoined by a further code word in order to represent a long sequence and this further code word is not recognized. In this case, the receiver is no longer able to assign the received code words correctly to the sequence lengths for the first color or the second color, as it can no longer recognize which characters form a code word. If, however, a line synchronizing word is provided at the beginning of each line, the effects of the fault can be limited to one line.
The German Published Application 2,264,090 discloses another process for facsimile coding in which, in accordance with a suitable algorithm, the color of each surface element is predicted from the color of a few spots which are already known to the receiver, and in which only those surface elements which have been incorrectly predicted are transmitted as a faulty image, in that the spaces between them are transmitted in a suitable sequence length code with a fixed code table. If, during this process, a sequence length is incorrectly transmitted, generally the remainder of the image content is adulterated.
From the German Published Application 2,031,646 a process is known in which a specific transmission format is used for the transmission of the code words. In accordance with this transmission format, in each case a given number of binary characters, which form the code words, are combined to form a block. Each block is preceded by a synchronization word which consists of a given sequence of binary characters. The synchronization word is followed by an address word which contains the address of the particular next surface element to be scanned, as an absolute address. If a transmission fault occurs, it can happen that, for example, the remainder of the contents of the block will be incorrectly reproduced. However, since the address of the next scanned surface element is transmitted after the next synchronization word, the following block is correctly reproduced.
As, however, the length of a block always contains a given number of binary characters which, for example, is between 256 and 267, in this process a block can extend over several lines. Therefore, the effects of a fault can extend over several lines. The next synchronization word is then transmitted at arbitrary points in a line and the remainder of the line is correctly reproduced from this point onwards. In this process the address words also require a plurality of binary characters as they form an absolute address.